Glyoxylates are chemical intermediates, for example, monomers suitably used as material for synthesizing sodium polyglyoxylate which is an effective builder component of a surface active agent.
In the case where the glyoxylates to be used as material for forming polymers contain impurities, particularly protic compounds such as water, alcohols, or carboxylic acids, a molecular weight of the resultant polymer tends to decrease. Therefore, when such glyoxylates are used as material for copolymers, the impurities have to be removed from the glyoxylates.
However, since the glyoxylates reversibly combine with water or alcohols in particular thereby forming hydrate, hemiacetal, or the like, to purify the glyoxylates is not easy.
The following purifying methods applicable to the glyoxylates have been proposed:
(1) executing distillation, with phosphoric anhydride added to a material containing glyoxylates;
(2) adding a higher alcohol with a boiling point of not lower than 180.degree. to a material containing glyoxylates, and executing distillation at a pressure not exceeding 800 mmHg (107 kPa) (the Japanese Publication for Laid-Open Patent Application No.62-178541/1987 (Tokukaisho 62-178541));
(3) distilling a mixture containing glyoxylates, glycolates, water, alcohols, and the like under a reduced pressure so that a content of water and alcohols decreases to less than 1 weight percent (wt. %), then, distilling it by adjusting so that a molar ratio of glycolate to glyoxylates becomes 1 to 1.4 (the Japanese Examined Patent Publication No.5-28694/1993 (Tokukohei 5-28694), the Japanese Publication for Laid-Open Patent Application No.60-97936/1985 (Tokukaisho 60-97936));
(4) when glyoxylates are produced from corresponding glycolates by oxidative dehydrogenation in a gaseous phase, adding an azeotropic agent such as methylene chloride, chloroform, n-pentane, cyclohexane, nonane, di-isopropyle ether, methyl ethyl ketone, benzene, toluene, or the like to a gaseous reaction mixture resulting on the oxidative dehydrogenation, and introducing the same to a distillation column (the Japanese Publication for Laid-Open Patent Application No.60-23345/1985 (Tokukaisho 60-23345));
(5) esterifying glyoxylic acid by reacting 1 mol of glyoxylic acid with 0.5 to 2 mol of lower alcohol in the presence of an azeotropic agent such as benzene or dichloroethane, and distilling the same after a concentration of water and alcohol in the reactive solution becomes not more than 10 wt. % each with respect to the resultant glyoxylates (the Japanese Publication for Laid-Open Patent Application No.61-50941/1985 (Tokukaisho 61-50941), the Japanese Examined Patent Publication No.4-66856/1992 (Tokukohei 4-66856)); and
(6) processing a reaction product obtained by oxidative dehydrogenation with respect to glycolates under a reduced pressure, then, supplying the reaction product thus processed to a multi-stage distillation column which holds a dense azeotropic agent such as methylene-dichloride, 1,1,1-trichloroethane, or benzene in the vicinity of its top, so that glyoxylates are taken from an intermediate point between the supplying part and the top part of the distillation column (the Japanese Examined Patent Publication No.7-42252/1995 (Tokukohei 7-42252), the Japanese Publication for Laid-Open Patent Application No.2-73040/1990 (Tokukaihei 2-73040), the Japanese Examined Patent Publication No.7-45435/1995 (Tokukohei 7-45435), the Japanese Publication for Laid-Open Patent Application No.1-254643/1989 (Tokukaihei 1-254643), the Japanese Examined Patent Publication No.5-28694/1993 (Tokukohei 5-28694)).
However, the method (1) is not preferable from an economic viewpoint, since phosphoric anhydride is consumed reacting with water or alcohols and hence it is hardly recovered. The method (2) has a drawback in that side reactions such as ester interexchange and the like may possibly occur. Moreover, since the method (3) requires excessive glycolates, the method (3) has a drawback in that productivity lowers due to the presence of the excessive glycolates. For these reasons, it is impossible to efficiently produce high-purity glyoxylates by the above methods (1) through (3).
Furthermore, as a result of various examinations by the inventors of the present application, the azeotropic agents used in the methods (4) and (5) exhibit insufficient performances, and high-purity glyoxylates are hardly obtained at a high yield with the use of the above azeotropic agents. Besides, in the case where a chemical compound such as benzene or trichloroethane is used as an azeotropic agent as in the case of the method (6), such a compound is toxic and it is difficult to deal with it.
In the aforementioned conventional methods, prior to a purification process whereby water is removed by azeotropy, coarse distillation for preliminarily concentrating glyoxylates in crude glyoxylates is carried out so that the purification is efficiently conducted, by utilizing a distillation column.
However, by the above conventional methods, the glyoxylates in the crude glyoxylates are, during coarse distillation, subjected to heating for a long period of time thereby becoming hydrolyzed. This causes a yield of the glyoxylates obtained through the purification process to decrease, thereby disenabling efficient production of high-purity glyoxylates.
Moreover, by the conventional methods, the glyoxylic acid produced through hydrolyzation is further resolved by a subsequent distillation process, thereby producing side products. Therefore, a content of the glyoxylates to be obtained lowers, leading to a drawback in that a purification efficiency of the glyoxylates deteriorates.